In recent years, a polymer electrolyte fuel cell (PEFC) using a polymer membrane represented by a membrane made of Nafion (manufactured by E.I. du Pont de Nemours and Company) has been investigated and developed as a fuel cell for a portable device, a movable body, a stationary instrument, and the like. These polymer membranes contain a large number of water molecules therein to exhibit sufficient proton conductivity necessary to operate as an electrolyte for fuel cells.
A problem with such a PEFC is a decrease in proton conductivity due to a decrease in the amount of water as a result of migration of water from an anode to a cathode, for example. The proton conductivity significantly decreases when a PEFC is operated at a temperature of 100° C. or more, at which water vaporization is abundant.
A method of reducing the thickness of the polymer membrane is used in order to control migration of water from an anode to a cathode or to reduce electrolyte resistance. However, reducing the membrane thickness causes a problem of permeation of raw material gas or liquid fuel. Because of the problem of water vaporization, the PEFC is operated at a temperature of 100° C. or less, usually about 80° C. At the same time, since it is necessary for the PEFC to correctly control the amount of water in the system, the system is prone to be complicated.
However, the following various problems have been pointed out when a PEFC is operated at a temperature of 100° C. or less.
(1) The electrode catalyst is poisoned by carbon monoxide contained in reforming hydrogen gas, which results in a decrease in catalytic activity.
(2) A large and complicated cooling device is required due to low heat exchange efficiency.
(3) A high output cannot be expected by operation in a low temperature region in which the chemical reaction efficiency is essentially low.
In order to overcome the problems associated with dry conditions without changing the basic technology and the application of PEFC and to raise the operating temperature as a means for solving the above problems (1) to (3), an electrolyte which operates at a high temperature of 150° C. or more in a no humid or low humid atmosphere is desired.
As candidate materials for these new solid electrolytes, porous glass, ammonium polyphosphate, cesium hydrogen sulfate, water-containing crystalline oxyacid, and the like have been proposed. None of these, however, exhibits high proton conductivity while satisfying the requirements of no humidity or low humidity operation at a high temperature.
Recently, inorganic and organic materials which exhibit high proton conductivity (10−2 S·cm−1 or more) at a mid-temperature in a range from 150 to 400° C. have been actively perused. One of the researches is a study on a metal phosphate shown by the formula MP2O7 (wherein M is a metal such as Si, Ge, Sn, or Ti) and a metal phosphate in which a portion of Sn4+ is doped with indium (In3+) ion to make a stoichiometric composition (see Non-patent Document 1). Although the proposed material has a possibility of being used as a proton conductor that can overcome the above-described problems in prior art, its electric conductivity is not yet sufficient and only a powdery sample has been obtained. As shown in FIG. 2, a formed body 2, which is obtained by press-forming a powder 1 of the metal phosphate MP2O7 mentioned above, may cause leakage R of a gaseous or liquid fuel X permeating through voids between the particles that form the formed body 2 or may cause cross-over of fuels if used as electrolyte.
[Non-patent Document 1] Electrochemistry, 73, pp. 846 to 850, No. 9 (2005), “4 Proton conductivity of MP2O7 solid electrolyte and application thereof”